Effects of energy demand in ischemic and in hypoxemic isolated rat hearts

Aim of this study was to assess the role of O2, lactate and energy demand in the regulation of myocardial work during severe dysoxia. For this purpose, we measured function and metabolism in isolated Langendorff-perfused rat hearts exposed to either ischemia or hypoxemia (matched for the O2 supply, 10% of baseline) with/out electrical stimulation. When hearts could adjust their HR, hypoxemia demanded more energy than ischemia (p<0.05) despite same O2 supply. Venous PO2 was 12±2 or 139±20 mmHg (p<0.0001), respectively, but VO2 was the same. After 10 min at HR=300 min-1, myocardial performance increased in ischemic but not in hypoxemic hearts P(v)O2 and were not affected by pacing. In contrast, both venous [lactate] and lactate production rate increased, but in ischemic hearts only. We conclude that ischemic hearts were downregulated while hypoxemic hearts were not. Likely, depressed washout of lactate during ischemia could offset the effects of O2 in severely dysoxic hearts. Anaerobic glycolysis provided the energy necessary to meet increased energy demand in ischemic hearts, but could not exploit this action in hypoxemic hearts probably because in these hearts it was already working near maximum.